Hearing tests are performed by presenting acoustic signals to a listener and asking the listener to indicate if the sound was audible. The sound level of the signal is varied to find the lowest levels that can be detected. Signals are typically presented by a transducer such as an earphone, a loudspeaker, or a bone conduction vibrator. The audiometer and the transducer used to present the signals are normally calibrated to ensure accurate and reliable measurements. For an acoustic transducer such as an earphone or a loudspeaker, calibration is usually performed with a microphone that receives the acoustic signal from the transducer and a sound level meter that is configured to receive and measure the signal from the microphone. A bone conduction transducer (also referred to as a bone conduction vibrator) is usually calibrated by converting the vibrations of the bone conduction vibrator into a measurable electrical signal.
One method of calibrating a bone conduction vibrator is to couple the vibrator to an artificial mastoid (e.g., Bruel & Kjaer Type 4930). The artificial mastoid is designed to mimic the mechanical impedance of the human head. The bone conduction vibrator is coupled to the artificial mastoid with one or more weights that provide a standard coupling force. The artificial mastoid transduces the mechanical vibration of the bone conduction vibrator to an electrical signal that is input to a sound level meter, which measures the level of the electrical signal. The measured voltage can then be expressed as the force level delivered by the vibrator. The American (ANSI S3.6-2004) and international (IEC 389.3-1994) audiometer standards provide standard reference equivalent threshold force levels (RETFL) and the bone vibrator and connected audiometer are calibrated so that the output of the bone vibrator is equal to the RETFL when the audiometer signal level control is set to 0 dB.
Another method of calibrating a bone conduction vibrator involves the use of an artificial mastoid simulator (e.g., Larson Davis AMC493). The bone conduction vibrator is coupled to the simulator in the same fashion as that used when calibrating with the artificial mastoid. The simulator transduces the vibratory force produced by the bone conduction vibrator into an acoustic signal that is measured by a microphone coupled to a sound level meter. The frequency responses of the microphone and the simulator are initially calibrated in accordance with empirically gathered data so that relationship between the acoustic sound pressure level produced by the simulator and the force level produced by the vibrator is known at each test frequency. This allows the audiometer and bone vibrator to be calibrated such that the output of the bone vibrator is equal to the RETFL when the audiometer signal level control is set for 0 dB.